ECONOMICAL PROCESS FOR ETAPORATlOIf. 375 



thongh important, was less interesting, and he thought no 

 more on the subject. The results, however, are too va- 

 luable, particularly at the present moment, to be kept 

 longer secret. 



As the air has the property of evaporating water at all Principle of the 

 temperatures, we have only to bring a large quantity into P*^^*^^*^ 

 contact with the wet matter we would dry, or the liquid we 

 would evaporate, to obtain a great effect in a little time. 



If we would obtain thii effect in a small space, which is 

 the object of almost all manufacturers, we must cause si 

 great deal of air to pass through this space in a little time by 

 producing a rapid current. A very simple machine, not 

 expensive, and well adapted to this purpose, is known by 

 the name of a centrifugal ventilator*. 



Thus in evaporation, without artificial heat, the caloric 

 necessary for the production of vapour will be that of the 

 atmospheric air : but it will be the free caloric, and not that 

 of combination, which combustibles alone can disengage. 

 To produce the union of the free caloric of the atmosphere 

 with water, simple contact alone is required; and this con- 

 tact may be effected by power. Power, therefore, is the 

 only expense required, to obtain this combination of water 

 with caloric. It will be shown presently, that tnis is much Advantages; 

 cheaper than fuel : besides, the former is always at our dis. 

 posal, while the latter becomes every day more expensive, 

 in consequence of the consumption of wood and of charcoal. 

 We have therefore a real adfantage, general and particular, 

 in the process of evaporating without artificial heat. 



Sometimes the atmospheric air is so damp, that it would Heated air may 

 dissolve but little water, and its being set ia motion would employed^ 

 profit but little. Such cases, however, are very rare; and 

 we see every day, even during rain, that the wind is drying.* 

 But to hasten evaporation, and regulate the process, the 

 air may be heated before it enters the evaporating machine : 

 it might be passed through a fire of charcoal, if the subject 

 under evaporation would be injured by smoke; or of aQT< 



♦ It is described by Desag\iliers in the Philosophical Transacv 

 tions for 1735 ; but he does not appear to have assigned the bit 

 possible dimensions to the machine. 



combustible 



